Method of driving discharge lamp, driving device, and projector

ABSTRACT

A method for driving a discharge lamp that supplies an AC current to a discharge lamp having a first electrode and a second electrode so as to produce discharge and to cause the discharge lamp to emit light includes the steps of, during a steady operation in which the AC current is supplied to the first electrode and the second electrode, when power of the same amount is fed to the first and second electrodes, and a tip portion of the first electrode becomes higher than a tip portion of the second electrode in temperature, changing the duty ratio of the AC current to be supplied between the first electrode and the second electrode in accordance with a predetermined pattern, and setting a current value when the first electrode operates as an anode during one cycle so as to be smaller than a current value when the second electrode operates as an anode during one cycle.

BACKGROUND

1. Technical Field

The present invention relates to a method for driving a discharge lamphaving a pair of electrodes, a driving device, and a projector includinga light source incorporated with such a discharge lamp.

2. Related Art

Heretofore, a lighting method for a high-intensity discharge lamp usinga single driving waveform is known. If lighting by a single drivingwaveform continues for a long time, the electrodes continuously have apredetermined temperature distribution for a long time, and as a result,asymmetry of the electrodes caused by a time-variant state change tendsto increase as time passes. For this reason, a plurality ofconcavo-convexes are generated around the tip portions of theelectrodes, and flicker occurs. In order to solve this problem, alighting method for a high-intensity discharge lamp is known in whichthe absolute value of an AC lamp current to be supplied to thehigh-intensity discharge lamp is set constant, and the AC lamp currentis pulse-width modulated (JP-T-2004-525496). Specifically, the ratio ofa pulse width of a positive pulse and a pulse width of a negative pulseis pulse-width modulated with a frequency lower than the lightingfrequency.

However, as described in JP-T-2004-525496, when the AC lamp current ispulse-width modulated, for example, if a light source is provided withan auxiliary mirror in order to efficiently converge light to be emittedforward, an electrode on the auxiliary mirror side may be excessivelydeteriorated, and a pair of electrodes may be deteriorated unevenly.

SUMMARY

An advantage of some aspects of the invention is that it provides amethod for driving a discharge lamp, which is capable of preventingelectrodes from wearing off unevenly, such a driving device, and aprojector using the same.

According to an aspect of the invention, there is provided a method fordriving a discharge lamp that supplies an AC current to a discharge lamphaving a first electrode and a second electrode so as to producedischarge and to cause the discharge lamp to emit light. The methodincludes, in a situation where a tip portion of the first electrodebecomes higher than a tip portion of the second electrode in temperaturewhen power of the same amount is fed to the first and second electrodesduring a steady operation in which the AC current is supplied to thefirst electrode and the second electrode, changing the duty ratio of theAC current to be supplied between the first electrode and the secondelectrode in accordance with a predetermined pattern, and setting acurrent value when the first electrode operates as an anode during onecycle so as to be smaller than a current value when the second electrodeoperates as an anode during one cycle.

With this driving method, during the steady operation, the duty ratio ofthe AC current to be supplied between the first electrode and the secondelectrode is changed in accordance with the predetermined pattern, andthe current value when the first electrode operates as an anode duringone cycle is set so as to be smaller than the current value when thesecond electrode operates as an anode during one cycle. Therefore, thefirst electrode can be prevented from being liable to be higher than thesecond electrode in temperature when power of the same amount is fed,and as a result, the first electrode can be prevented from beingdeteriorated earlier than the second electrode.

According to a specific example or respect of the invention, in theabove-described driving method, a primary reflecting mirror may bedisposed on the second electrode side to reflect a light beam generatedby discharge between the first electrode and the second electrode so asto be emitted toward a region to be illuminated, and an auxiliaryreflecting mirror may be disposed on the first electrode side so as tobe opposite the primary reflecting mirror to reflect a light beam froman inter-electrode space between the first electrode and the secondelectrode toward the inter-electrode space. In this case, the firstelectrode on the auxiliary reflecting mirror side can be prevented frombecoming higher than the second electrode on the primary reflectingmirror in temperature. The reason why the first electrode on theauxiliary reflecting mirror side has a comparatively high temperature isconsidered that the first electrode is located closer to the auxiliaryreflecting mirror and is likely to be more exposed to radiation heatfrom the auxiliary reflecting mirror, or that cooling wind flowingaround the light-emitting tube is blocked by the auxiliary reflectingmirror, and cooling efficiency is lowered on a side of the firstelectrode covered with the auxiliary reflecting mirror, that is, in ahemisphere in which the first electrode is accommodated.

According to another example of the invention, the AC current mayconstantly provide a difference of a predetermined current value betweenthe absolute value of the current value when the first electrodeoperates as an anode during one cycle and the absolute value of thecurrent value when the second electrode operates as an anode during onecycle. In this case, the first electrode and the second electrode aremaintained at the same temperature, and in terms of thermoelectronicemission, the balance of both electrodes is maintained, therebymaintaining stable arc discharge. In addition, the driving waveform canbe simply formed by superimposing a DC current on a square wave.

According to yet another example of the invention, the AC current mayconstantly provide a difference of a predetermined ratio between theabsolute value of the current value when the first electrode operates asan anode during one cycle and the absolute value of the current valuewhen the second electrode operates as an anode during one cycle. In thiscase, the first electrode and the second electrode are maintained at thesame temperature, and in terms of thermoelectronic emission, the balanceof both electrodes is maintained, thereby maintaining stable arcdischarge.

According to yet another example of the invention, the current value maybe controlled such that the average power value during one cycle of theAC current substantially becomes identical to the average power valueduring one cycle of the predetermined pattern. In this case, powerduring emission is maintained constant, and thus a change in luminancecorresponding to the cycle of the predetermined pattern is difficult tobe generated.

According to yet another example of the invention, the current value maybe controlled only at a polarity having a larger duty ratio during onecycle of the AC current. In this case, the current value at a polarityhaving a smaller duty ratio is maintained constant, and the emissionstate of the discharge lamp is stably maintained.

According to an aspect of the invention, there is provided a drivingdevice that supplies an AC current to a discharge lamp having a firstelectrode and a second electrode so as to produce discharge and to causethe discharge lamp to emit light. The driving device includes a currentdriving circuit that, in a situation where a tip portion of the firstelectrode becomes higher than a tip portion of the second electrode intemperature when power of the same amount is fed to the first and secondelectrodes during a steady operation in which the AC current is suppliedto the first electrode and the second electrode, changes the duty ratioof the AC current to be supplied between the first electrode and thesecond electrode in accordance with a predetermined pattern, and setsthe absolute value of a current value when the first electrode operatesas an anode during one cycle so as to be smaller than the absolute valueof a current value when the second electrode operates as an anode duringone cycle.

With this driving device, during the steady operation, the drivingcircuit changes the duty ratio of the AC current to be supplied betweenthe first electrode and the second electrode in accordance with thepredetermined pattern, and sets the current value when the firstelectrode operates as an anode during one cycle so as to be smaller thanthe current value when the second electrode operates as an anode duringone cycle.

Therefore, the first electrode can be prevented from being liable to behigher than the second electrode in temperature when power of the sameamount is fed, and as a result, the first electrode can be preventedfrom being deteriorated earlier than the second electrode.

According to an aspect of the invention, a projector includes a lightsource device that is driven by the above-described driving method andemits light, a light modulation device that receives a light beam fromthe light source device, and a projection optical system that projectsan image formed by the light modulation device.

With this projector, the above-described light source device is used.Therefore, both electrodes of the light source device can be preventedfrom being deteriorated, or the electrodes can be prevented from beingdeteriorated unevenly. As a result, the projection luminance of theprojector can be maintained over a long period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a sectional view illustrating a light source device accordingto an embodiment of the invention.

FIG. 2 is a block diagram showing the configuration of a current drivingdevice incorporated into a light source device.

FIG. 3 is an enlarged sectional view illustrating a body portion of alight-emitting tube.

FIGS. 4A to 4C are enlarged views illustrating a repair process ofelectrodes by a light source driving device.

FIG. 5 is a diagram illustrating the waveform of an AC current to besupplied to both electrodes.

FIGS. 6A and 6B are diagrams illustrating an AC current to be suppliedto both electrodes.

FIG. 7 is a diagram illustrating a modification of an AC current to besupplied to both electrodes.

FIG. 8 is a diagram illustrating a modification of an AC current to besupplied to both electrodes.

FIG. 9 is a flowchart illustrating the operation of a light sourcedriving device.

FIG. 10 is a diagram illustrating a projector incorporated with a lightsource device.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a light source device incorporated with a driving devicefor a discharge lamp according to an embodiment of the invention will bedescribed with reference to the drawings.

FIG. 1 is a sectional view conceptually illustrating the structure of alight source device 100. In the light source device 100, a light sourceunit 10 includes a discharge emission-type light-emitting tube 1 servingas a discharge lamp, an elliptical reflector 2 serving as a primaryreflecting mirror, and a spherical auxiliary mirror 3 serving as anauxiliary reflecting mirror. A light source driving device 70 includes,an electrical circuit, serving as a driving device for a discharge lamp,which supplies an AC current to the light source unit 10 so as to causethe light source unit 10 to emit light in a desired state.

In the light source unit 10, the light-emitting tube 1 is formed of atransmissive silica glass tube having a central portion thereof swollenspherically. The light-emitting tube 1 includes a body portion 11 thatis a sealed body configured to emit light for illumination, and firstand second seal portions 13 and 14 that extend along an axis passingthrough both ends of the body portion 11.

In a discharge space 12 formed in the body portion 11, a tip portion ofa first electrode 15 made of tungsten and a tip portion of a secondelectrode 16 made of tungsten are disposed so as to be spaced at apredetermined distance from each other, and a compound containing raregas and halogen and mercury are filled. In respective seal portions 13and 14 extending from both ends of the body portion 11, metal foils 17 aand 17 b made of molybdenum are filled, respectively. The metal foils 17a and 17 b are electrically connected to base portions of the first andsecond electrodes 15 and 16 provided in the body portion 11,respectively. The seal portions 13 and 14 are sealed airtight from theoutside by a glass material. If power is supplied to the light-emittingtube 1 through lead wires 18 a and 18 b connected to the metal foils 17a and 17 b by the light source driving device 70 in the form of ACpulses, arc discharge is generated between a pair of electrodes 15 and16, and the body portion 11 emits light with high-intensity.

The auxiliary mirror 3 is located close to the body portion 11 of thelight-emitting tube 1 and covers a substantially half of the bodyportion 11 on a front side in a beam emission direction on which thefirst electrode 15 is present. The auxiliary mirror 3 is a mold productmade of silica glass as a single body. The auxiliary mirror 3 includesan auxiliary reflecting portion 3 a that gets the light beam emittedfrom the body portion 11 of the light-emitting tube 1 toward the frontback to the body portion 11, and a support portion 3 b that is fixed tothe periphery of the first seal portion 13 in a state of supporting abase portion of the auxiliary reflecting portion 3 a. The supportportion 3 b has the first seal portion 13 inserted therein and holds theauxiliary reflecting portion 3 a in a state of being aligned with thebody portion 11.

The reflector 2 is disposed so as to be opposite a substantially half ofthe body portion 11 of the light-emitting tube 1 on a rear side in thebeam emission direction on which the second electrode 16 is present. Thereflector 2 is a mold product made of crystallized glass or silica glassas a single body. The reflector 2 includes a neck portion 2 a throughwhich the second seal portion 14 of the light-emitting tube 1 isinserted, and a primary reflecting portion 2 b that has an ellipticallycurved surface expanding from the neck portion 2 a. The neck portion 2 ahas the second seal portion 14 inserted therein and holds the primaryreflecting portion 2 b in a state of being aligned with the body portion11.

The light-emitting tube 1 is disposed along a system optical axis OAcorresponding to an axis of rotational symmetry or an optical axis ofthe primary reflecting portion 2 b such that an emission center Obetween the first and second electrodes 15 and 16 inside the bodyportion 11 becomes substantially identical to the position of a firstfocus F1 of the elliptically curved surface of the primary reflectingportion 2 b. When the light-emitting tube 1 is lighted, light beamsemitted from the arc around the emission center O of the body portion 11are reflected by the primary reflecting portion 2 b or reflected by theauxiliary reflecting portion 3 a and then further reflected by theprimary reflecting portion 2 b, and are formed as light beams convergedat the position of a second focus F2 of the elliptically curved surface.That is, the reflector 2 and the auxiliary mirror 3 have reflectingcurved surfaces substantially axisymmetric with respect to the systemoptical axis OA, and the pair of electrodes 15 and 16 are disposed suchthat the electrode axis becomes substantially identical to the systemoptical axis OA, which is the center of the axis thereof.

The light-emitting tube 1 is manufactured by a shrink seal whichsupports the first and second electrodes 15 and 16 individually fixed tothe tips of the metal foils 17 a and 17 b inside a silica glass tube,and in which the silica glass tube is heated from the periphery thereofby a burner at portions corresponding to both seal portions 13 and 14,softened, and contracted. The auxiliary mirror 3 is fixed to thelight-emitting tube 1 by injecting, filling, and solidifying aninorganic adhesive C in a state where the support portion 3 b isinserted through the first seal portion 13 of the light-emitting tube 1.The light-emitting tube 1 is fixed to the reflector 2 by injecting,filling, and solidifying an inorganic adhesive C in a state where thesecond seal portion 14 is inserted into the neck portion 2 a of thereflector 2.

FIG. 2 is a block diagram schematically showing the configuration of thelight source driving device 70 for turning on the light source unit 10shown in FIG. 1 in a desired state.

The light source driving device 70 generates an AC current for producingdischarge between a pair of electrodes 15 and 16 shown in FIG. 1, andcontrols the supply state of the AC current to both electrodes 15 and16. The light source driving device 70 includes a lighting device 70 a,a control device 70 b, and a DC/DC converter 70 c. Here, an examplewhere the light source driving device 70 uses an external power supplywill be described. That is, the light source driving device 70 isconnected to an AC/DC converter 81, and the AC/DC converter 81 isconnected to a commercial power supply 90. The AC/DC converter 81converts an AC current, which is supplied from the commercial powersupply 90, into a DC current.

The lighting device 70 a is a circuit that is driven to turn on thelight source unit 10 shown in FIG. 1. The lighting device 70 a adjusts adriving waveform output from the light source driving device 70. Here,the driving waveform has factors, such as frequency of output current orvoltage, amplitude, duty ratio, ratio of positive and negativeamplitudes, waveform characteristics, and the like. A driving currenthaving an arbitrary waveform characteristic, such as a square wave, asuperimposed wave obtained by superimposing a triangular wave on such asquare wave, or the like is output from the lighting device 70 a to thelight source unit 10.

The control device 70 b is a circuit unit that includes a microcomputer,a memory, a sensor, an interface, and the like, and is driven by anappropriate driving voltage generated by the DC/DC converter 70 cserving as a power supply. The control device 70 b includes a drivingcontrol section 74 that controls the operation state of the lightingdevice 70 a, a determination section 75 that determines the state of thelight-emitting tube 1, and a data storage section 76 that stores variouskinds of information regarding the operation mode of the lighting device70 a, that is, power feed conditions and the like.

The driving control section 74 operates in accordance with a programstored in the data storage section 76 or the like. The driving controlsection 74 selects one of a power feed condition for an initialoperation and a power feed condition for a steady operation stored inthe data storage section 76 in accordance with the current state of thelight-emitting tube 1, and causes the lighting device 70 a to executethe initial operation or the steady operation in accordance with theselected power feed condition. The driving control section 74 functionsas a current driving circuit that feeds power to the light-emitting tube1 to execute a necessary lighting operation, together with the lightingdevice 70 a. In this embodiment, an operation to supply normal energy tothe second electrode 16 and the first electrode 15 is called the steadyoperation, and an operation, different from the steady operation whenlighting starts before the steady operation is executed, to supplyenergy to the second electrode 16 and the first electrode 15 is calledthe initial operation.

The determination section 75 determines the operation condition of thelight-emitting tube 1, such as a deterioration level, on the basis ofthe state of the light-emitting tube 1, that is, a cumulative lightingtime of the light-emitting tube 1, an inter-electrode voltage to thelight-emitting tube 1, and the like.

The data storage section 76 stores a plurality of power feed conditionsfor the initial operation as the initial operation modes of thelight-emitting tube 1 and a plurality of power feed conditions for thesteady operation as the steady operation modes of the light-emittingtube 1, in addition to the program for the operation of the drivingcontrol section 74. Specifically, the data storage section 76 storesvarious parameters, such as set values of a current value, a frequency,and the like during the initial operation, for example, when initiationor during a rising time period. The data storage section 76 also storesvarious parameters regarding a current value, a lighting frequency, aduty ratio, modulation contents of the duty ratio, a ratio of positiveand negative amplitudes, a triangular wave jump rate, and the likeduring the steady operation. The modulation contents of the duty ratioinclude a variable range of the duty ratio, a division period, amodulation frequency, a correction amount, and the like as parameters.

FIG. 3 is an enlarged sectional view illustrating the inside of the bodyportion 11 of the light-emitting tube 1 shown in FIG. 1. FIGS. 4A to 4Care conceptual views illustrating a repair process of both electrodes 15and 16. As shown in the drawings, the first and second electrodes 15 and16 in the body portion 11 individually includes tip portions 15 a and 16a, large diameter portions 15 b and 16 b, and shaft portions 15 c and 16c.

In the case of the first electrode 15 shown in FIG. 4A, a plurality ofminute concavo-convexes 63 are formed in a surface on a tip side of thetip portion 15 a. In this case, a phenomenon that a discharge startpoint is moved between the tip portion 15 a and the concavo-convexes 63,that is, flicker or arc jump occurs. Here, flicker means a phenomenonthat the discharge start point is continuously moved, and arc jump meansa phenomenon that the discharge start point is completely moved from anoriginal discharge start point. Flicker or arc jump causes displayflicker or deterioration of illuminance.

It is assumed that in order to prevent flicker or arc jump after orbefore the fact, during the steady operation of the light-emitting tube1, as described below in detail, the first and second electrodes 15 and16 are alternately subject to a cyclic repair process.

In an example shown in FIG. 4B, for example, the duty ratio of the ACcurrent indicating the ratio of an operation time of the first electrode15 as an anode is set so as to be appropriately larger than 50% duringone cycle. When this happens, the temperature of the tip of the firstelectrode 15 rises, and the concavo-convexes 63 of FIG. 4A are melted,thereby forming a molten portion 64. Specifically, if the duty ratio(hereinafter, also referred to as an AC current duty ratio) indicatingthe ratio of an operation time as an anode during one cycle of the ACcurrent included in the current waveform to be supplied to a pair ofelectrodes 15 and 16 is maintained so as to be larger than 50% for apredetermined time, the rise in temperature of the tip of an electrode(in FIG. 4B, the first electrode 15) serving as an anode when the anodeduty ratio is larger than 50% can be appropriately controlled. Theadjustment of the amount of rise in temperature of the tip of the firstelectrode 15 ensures that the molten portion 64 can be formed in thesurface of the tip portion 15 a while the tip portion 15 a substantiallyslightly remains, and the concavo-convexes 63 can become flat. After thetip of the first electrode 15 is sufficiently heated, as shown in FIG.4C, if the AC current duty ratio indicating the ratio of the operationtime of the first electrode 15 as an anode during one cycle ismaintained so as to be smaller than 50% for a predetermined time, thefall in temperature of the tip of the first electrode 15 can beappropriately controlled. As a result, the molten portion 64 of FIG. 4Bis gradually cooled. The cooled molten portion 64 is solidified, and asshown in FIG. 4C, the tip portion 15 a is maintained in a taper shapehaving an adequately large size.

In the foregoing description, repair driving has been described focusingon the first electrode 15, but for the second electrode 16, the samerepair driving may be performed simultaneously. That is, in the stateshown in FIG. 4C, the temperature of the tip of the second electrode 16rises, and the molten portion 64 is formed. Next, if the state of FIG.4C returns to the state of FIG. 4B, the temperature of the tip of thesecond electrode 16 falls, the cooled molten portion 64 is solidified.Thus, the shape of the tip portion 16 a is maintained in a taper shape.

That is, by alternately repeating the state of FIG. 4B where the firstelectrode 15 serving as an anode is heated and the second electrode 16is cooled, and the state of FIG. 4C where the second electrode 16serving as an anode is heated and the first electrode 15 is cooled, thefirst and second electrodes 15 and 16 are alternately repaired.Therefore, both electrodes 15 and 16 can be prevented from beingdeteriorated, and the lifespan of the light-emitting tube 1 can beextended.

Returning to FIG. 3, during the steady operation in which thelight-emitting tube 1 operates in a substantially stable state, an arcAR is formed in an inter-electrode space between the tip portions 15 aand 16 a of a pair of electrodes 15 and 16 by arc discharge. The arc ARand the periphery thereof become high in temperature. For this reason,convection AF flowing from the arc AR upward is formed in the dischargespace 12. The convection AF hits a top portion 11 a of the body portion11, is moved along an upper half portion lib thereof, passes through theshaft portions 15 c and 16 c of both electrodes 15 and 16, and issettled while being cooled. The settled convection AF is further settledalong a lower half portion 11 c of the body portion 11, collides againsteach other below the arc AR, and rises so as to return to the arc ARupward. That is, the convection AF is formed and circulated around bothelectrodes 15 and 16, but the convection AF may include an electrodematerial melted and evaporated by the arc AR. The electrode material maybe topically accumulated or segregated in the shaft portions 15 c and 16c by normal convection and may be grown in the form of needle, andunintended discharge may be produced toward the upper half portion 11 b.Unintended discharge causes deterioration of an inner wall of the bodyportion 11 and shortening of the lifespan of the light-emitting tube 1.In addition, when lighting by a single driving waveform continues for along time, the electrodes continuously have a predetermined temperaturedistribution for a long time, and as a result, asymmetry of theelectrodes caused by a time-variant state change tends to increase astime passes. For this reason, the duty ratio of the AC current to besupplied between the first and second electrodes 15 and 16 is slowlychanged so as to be waved, and the temperature distribution of theelectrodes is cyclically changed. Then, the electrodes are preventedfrom being deteriorated unevenly, and normal convection AF is preventedfrom being formed in the discharge space 12 due to a difference intemperature, several hundred K, between the left and right electrodes.Specifically, the duty ratio of the current waveform is cyclicallychanged in a cycle sufficiently larger than the cycle of the AC currentincluded in the current waveform to be supplied to a pair of electrodes15 and 16. In this case, in order to increase the change of theconvection AF, a pattern for changing the duty ratio of the AC currentto be supplied to the electrodes 15 and 16 is maintained at a pluralityof different values for a predetermined period of time or more in aplurality of division periods constituting a cyclic period (that is,modulation cycle) regarding the change of the duty ratio. That is, theduty ratio of the AC current in the current waveform to be supplied tothe electrodes 15 and 16 is changed in a stepwise manner by a discretevalue, and is cyclically increased or decreased over a sufficiently longperiod of time. By adjusting the variable range of the AC current dutyratio and the displacement cycle, the first and second electrodes 15 and16 shown in FIGS. 4B and 4C are repaired simultaneously.

A specific driving condition will now be described. It is assumed thatthe frequency of the AC current to be supplied to both electrodes 15 and16 is, for example, in a range of about 60 to 500 Hz. In addition, thevariable range of the duty ratio indicating the ratio of the operationtime of the first electrode 15 as an anode during one cycle is, forexample, in a range of 30%:70% to 70%:30%. Each of the division periodsconstituting the current cycle of the duty ratio is set so as to be, forexample, one second or more, and the duty ratio in each of the divisionperiods is maintained constant. Here, the modulation cycle is dividedinto, for example, about eight periods, and thus the modulation cycle ofthe duty ratio including all the division periods each being set so asto be one second becomes, for example, eight seconds. With such amodulation pattern, the thermal states of both electrodes 15 and 16 andthe periphery thereof can be slowly changed over such a long span thatthe convection AF is influenced. Therefore, normal convection AF can beavoided from being formed inside the body portion 11 of thelight-emitting tube 1. As a result, the electrode material can beprevented from being grown in the form of needle at unintended places ofboth electrodes 15 and 16, and the shapes of both electrodes 15 and 16can be prevented from being rapidly deteriorated.

FIG. 5 is a graph illustrating an example of the modulation patternregarding the AC current duty ratio of the current waveform to besupplied to a pair of electrodes 15 and 16. The horizontal axisrepresents a time, and the vertical axis represents an AC current dutyratio. As will be apparent from FIG. 5, the duty ratio of the AC currentto be supplied to both electrodes 15 and 16 changes at a predeterminedratio if the division cycle changes, and cyclically increases ordecreases in a modulation cycle Tm. The modulation cycle Tm includes afirst half cycle H1 in which an anode period of the first electrode 15becomes relatively long, and a second half cycle H2 in which an anodeperiod of the second electrode 16 becomes relatively long. As themodulation pattern of the duty ratio of the AC current, a pattern isused in which one cycle is divided into 8 division periods P1 to P8, andthe duty ratio during the anode period of the first electrode 15 ischanged five times in three steps at an interval of 10% in a range of30% to 70%. Specifically, during the division period P1 of the firsthalf cycle H1, the duty ratio is maintained at 50% over a predeterminedperiod of time (for example, about one second), during the divisionperiod P2, the duty ratio is maintained at 60% over the samepredetermined period of time, during the division period P3, the dutyratio is maintained at 70% over the same predetermined period of time,and during the division period P4, the duty ratio is maintained at 60%over the same predetermined period of time. Subsequently, during thedivision periods P5, P6, P7, and P8 constituting the second half cycleH2, the duty ratio is maintained at 50, 40, 30, and 40% over apredetermined period of time, respectively. In this way, if the dutyratio is increased or decreased in a stepwise manner, heat shock againstthe tip portions 15 a and 16 a of both electrodes 15 and 16 can bereduced. The maximum value DM1 of the duty ratio during the anode periodof the first electrode 15 in the first half cycle H1 in which the firstelectrode 15 serves as an anode, and the maximum value DM2 of the dutyratio during the anode period of the second electrode 16 in the secondhalf cycle H2 in which the second electrode 16 serves as an anode areset so as to be identical, for example, 70%.

Hereinafter, the fact that the first electrode 15 on the auxiliarymirror 3 side becomes higher than the second electrode 16 on thereflector 2 side in temperature will be described. First, the firstelectrode 15 is located closer to the auxiliary mirror 3 than the secondelectrode 16, and accordingly, the first electrode 15 is liable to beexposed to radiation heat from the auxiliary mirror 3. For this reason,the first electrode 15 is liable to become relatively higher than thesecond electrode 16 in temperature. The light source unit 10 is cooledto an adequate temperature by cooling wind from a cooling device (notshown), but cooling efficiency tends to be lowered in the hemisphere ofthe body portion 11 of the light-emitting tube 1 covered with theauxiliary mirror 3. Therefore, the first electrode 15 is liable torelatively become higher than the second electrode 16 in temperature. Asdescribed above, the first electrode 15 on the auxiliary mirror 3 sideis liable to become higher than the second electrode 16 in temperature,and thus the deterioration rate of the first electrode 15 is increased.For this reason, unevenness occurs in the AC current to be supplied tothe first and second electrodes 15 and 16. Specifically, the currentvalue I1, which is the maximum value or the average value when the firstelectrode 15 operates as an anode during one cycle, is set so as to besmaller than the current value I2, which is the maximum value or averagevalue when the second electrode 16 operates as an anode during onecycle. As a method for generating such a driving waveform, asuperimposed current in which a DC component having a negativecorrection amount d is superimposed on an AC component having the samepositive and negative amplitudes, A0, is supplied between bothelectrodes 15 and 16. When this happens, the current value when thefirst electrode 15 serves as an anode becomes I1=A0−d (the absolutecurrent value |I1|=A0−d), and the current value when the secondelectrode 16 serves as an anode becomes I2=−A0−d (the absolute currentvalue |I2|=A0+d). Therefore, power to be supplied to the first electrode15 relatively decreases, as compared with power to be supplied to thesecond electrode 16, and thus the rise in temperature of the firstelectrode 15 on the auxiliary mirror 3 side can be suppressed.Specifically, the correction amount d on the first electrode 15 side isset so as to be, for example, about 20% of the amplitude A0 of the ACcomponent. Therefore, the first electrode 15 on the auxiliary mirror 3side can be prevented from wearing off unevenly, while the luminance ofthe arc AR can be ensured. The correction amount d of the current by theDC component is not limited to the value 20%, but it may beappropriately adjusted in accordance with how the first electrode 15 isliable to have a higher temperature.

In the above description, the fact that the first electrode 15 of theauxiliary mirror 3 side has a higher temperature means that during thesteady operation, power of the same amplitude or duty ratio is fed tothe first electrode 15 and the second electrode 16, and the firstelectrode 15 becomes higher than the second electrode 16 in temperature.For example, it means a case in which the maximum temperature during onecycle of the AC current to be supplied between both electrodes 15 and 16relatively becomes high on the first electrode 15 side, a case in whichthe average temperature during one cycle relatively becomes high on thefirst electrode 15 side, or a case in which the average temperatureduring an anode period relatively becomes high on the first electrode 15side.

FIG. 6A is a graph illustrating a driving waveform which is actuallysupplied to a pair of electrodes 15 and 16. The horizontal axisrepresents a time, and the vertical axis represents a current value. Asshown in FIG. 6A, a square wave current having a predetermined lightingfrequency corresponding to the AC cycle Ta is supplied to bothelectrodes 15 and 16. In this case, though described below in detail,the square wave current is not maintained to a predetermined amplitude,that is, the absolute current values I1 and I2, but the square wavecurrent slightly increases or decreases by an adjustment amount Δd ofpower from the standard value and has the absolute current values I1′and I2′ that vary in a stepwise manner.

In FIG. 6A, with respect to the waveforms W1, W2, W3, and W4individually included in the division periods P1, P2, P3, and P4constituting the first half cycle H1 (see FIG. 5), the duty ratio of theAC current during each division period is maintained constant. Then,each time the division periods P1, P2, P3, and P4 are switched, the dutyratio indicating the ratio of the operation time of the first electrode15 as an anode during one cycle of a corresponding one of the waveformsW1, W2, W3, and W4 increases to 50% to 70% in a stepwise manner,reversely decreases to 70% to 30% in a stepwise manner, and finallyincreases to 30% to 50% in a stepwise manner, thereby returning to theeven state. Meanwhile, with respect to the waveforms W5, W6, W7, and W8individually included in the division periods P5, P6, P7, and P8constituting the second half cycle H2 (see FIG. 5), the duty ratio ofthe AC current during each division period is maintained constant. Then,each time the division periods P5, P6, P7, and P8 are switched, the dutyratio of a corresponding one of the waveforms W5, W6, W7, and W8temporarily decreases in a stepwise manner, reversely increases in astepwise manner, and finally increases in a stepwise manner, therebyreturning to the even state.

FIG. 6B is a graph illustrating power which is actually to be suppliedto a pair of electrodes 15 and 16. In this case, consequently, power ismaintained at a predetermined value BO, as indicated by a solid line. Adotted line represents a case in which the amplitude is not adjusted,that is, the amplitude is not increased or decreased, unlike the drivingwaveform of FIG. 6A, and represents power when the absolute currentvalue I1 or I2 is maintained at a predetermined value. When theamplitude is not adjusted, power increases or decreases in themodulation cycle Tm in accordance with switching of the division periodsP1 to P8. The change in power counteracts the rated operation, andmeans, though not noticeable, a change in luminance. For this reason, inorder to suppress the change in power, with respect to the waveforms W2,W3, W4, W6, W7, and W8 of FIG. 6A, the values obtained by slightlydecreasing or increasing the standard current values I1=A0−d andI2=−A0−d by the adjustment amount Δd are set as the current values I1′and I2′. In the above description, the adjustment amount Δd of each ofthe waveforms W2, W3, W4, W6, W7, and W8 becomes identical when thefirst electrode 15 serves as an anode during one cycle and when thesecond electrode 16 serves as an anode during one cycle. That is, thesame difference of current value is constantly provided between theabsolute current values I1 and I1′ when the first electrode 15 operatesas an anode, and the absolute current values I2 and I2′ when the secondelectrode 16 operates as an anode, regardless of the change in the dutyratio or the DC component.

FIG. 7 is a graph illustrating a modification of the driving waveform ofFIG. 6A. In this case, while the waveform W1 of the division period P1has the standard absolute current values I1 and I2, for example, thewaveform W3 of the division period P3 has the absolute current valuesI1′ and I2′ that are slightly increased from the standard absolutecurrent values I1 and I2 by an adjustment amount Δe, respectively. Theratio of the adjustment amount Δe when the first electrode 15 serves asan anode during one cycle and the adjustment amount Δe when the secondelectrode 16 serves as an anode is identical to the ratio of thestandard absolute current values I1 and I2. That is, the ratio of bothabsolute current values I1′ and I2′ after adjustment is maintained tothe ratio of the original absolute current values I1 and I2, and theabsolute current values when both electrodes 15 and 16 operate as ananode constantly have a difference of a predetermined ratio, regardlessof the change in the duty ratio or the DC component.

FIG. 8 is a graph illustrating another modification of the drivingwaveform of FIG. 6A. In this case, while the waveform W1 of the divisionperiod P1 has the standard absolute current values I1 and I2, forexample, the waveform W3 of the division period P3 has the absolutecurrent value I1′, which is slightly increased from the standardabsolute current value I1 by an adjustment amount Δf, at a polarity atwhich a lighting cycle is relatively long, that is, when the firstelectrode 15 side serves as an anode. Here, the adjustment amount Δf isset to a value sufficient to suppress the change in power to be suppliedto both electrodes 15 and 16. With the adjustment of the current valueat a polarity at which a lighting cycle is relatively long, flicker dueto a change in the light amount can be reduced by a comparatively smalladjustment amount Δf.

Referring to the waveforms shown in FIGS. 6B, 7, and the like, the ACcurrent is supplied to both electrodes 15 and 16, and the duty ratio ofthe AC current slowly changes to be positive and negative in acomparatively long cycle. In addition, the DC component is superimposedon the AC component while the change in power is suppressed, and thuspower to be supplied to the first electrode 15 is set so as to berelatively larger than power to be supplied to the second electrode 16.Therefore, the first and second electrodes 15 and 16 can be preventedfrom wearing off unevenly while the repair process is performed, and therelative rise in temperature of the first electrode 15 on the auxiliarymirror 3 side can be suppressed.

In the operation pattern of the current waveform shown in FIGS. 6A and6B, and the like, the lighting frequency and the like of the current tobe supplied to both electrodes 15 and 16 does not need to be maintainedconstant, and different lighting frequencies can be assigned to thedivision periods P1, P2, P3, . . . . The number of division periods andthe increase or decrease pattern of the duty ratio can also be changed.In addition, when the first electrode 15 serves as an anode or when thesecond electrode 16 serves as an anode, a superimposed wave in which atriangular wave is superimposed on a square wave can be used while theaverage current value is maintained. Let the average current value ofthe square wave AC component before being superimposed be A0, and thepeak value after being superimposed be A1, with the adjustment of athus-defined triangular wave jump rate A1/A0, the tip of the anode canbe sufficiently melted, and flicker in the cathode can be suppressed.

In the operation pattern of the current waveform shown in FIGS. 6A and6B, and the like, the set values of the lighting frequency, the dutyratio, the modulation contents of the duty ratio, a difference betweenpositive and negative amplitudes, the ratio of positive and negativeamplitudes, the triangular wave jump rate, and the like can bedynamically changed as occasion demands on the basis of informationregarding a deterioration level obtained by the determination section75, for example, how both electrodes 15 and 16 wear off. For example,when both electrodes 15 and 16 wear off, the lighting frequency and thecurrent value temporarily increase or decrease, thereby maintaining theshapes of the tip portions 15 a and 16 a of both electrodes 15 and 16.In addition, by increasing the maximum values DM1 and DM2 of the dutyratio, the electrodes which are deteriorated as time passes can bereliably melted, and the shapes of the tips can be preferablymaintained.

FIG. 9 is a flowchart illustrating the operation of the light sourcedriving device 70. The control device 70 b reads out adequate initialdriving data necessary for starting to turn on the light-emitting tube 1from a driving control table stored in the data storage section 76 (StepS11).

Next, the control device 70 b controls the lighting device 70 a on thebasis of a power feed condition for an initial operation read in StepS11, and controls the initial operation including initiation and risingof the light-emitting tube 1 (Step S12).

Next, the control device 70 b reads out adequate normal driving datanecessary for maintaining the emission state of the light-emitting tube1 from the driving control table stored in the data storage section 76(Step S13). Specifically, the set values of the lighting frequency, theduty ratio, the modulation contents of the duty ratio, the differencebetween positive and negative amplitudes, the ratio of positive andnegative amplitudes, the triangular wave jump rate, and the like duringthe steady operation are read out. In this case, a lighting waveform,such as the lighting frequency, the current value regarding positive andnegative amplitudes, and the like, and a driving pattern including themodulation range of the duty ratio, the division period, the modulationcycle, and the like is selected on the basis of information regarding adeterioration level obtained by the determination section 75, forexample, how both electrodes 15 and 16 wear off.

Next, the control device 70 b controls the steady operation of thelight-emitting tube 1 of the lighting device 70 a on the basis of apower feed condition for a steady operation read in Step S13 (Step S14).A specific operation is illustrated in FIGS. 4A to 4C, 5, 6A and 6B, 7,and 8.

The determination section 75 determines whether or not an interruptrequest signal for requesting the end of the lighting operation of thelight source unit 10 is input during the steady operation (Step S15).When the interrupt request signal is input, information regarding thecurrent state of the light-emitting tube 1, such as a current cumulativelighting time, a voltage being currently supplied to the light-emittingtube 1, and the like, is recorded in the data storage section 76, andthen a lighting-out operation is executed.

As will be apparent from the foregoing description, according to thelight source device 100 of this embodiment, during the steady operationin which the light-emitting tube 1 is in rated operation, the duty ratioof the AC current to be supplied between the first and second electrodes15 and 16 is changed in accordance with a cycle pattern by the lightingdevice 70 a, which is operated under the control of the control device70 b, and the current value I1 when the first electrode 15 operates asan anode during one cycle is set so as to be smaller than the currentvalue I2 when the second electrode 16 operates as an anode during onecycle. Therefore, the first electrode 15 can be prevented from beingliable to be higher than the second electrode 16 in temperature whenpower of the same amount is fed, and as a result, the first electrode 15can be prevented from being deteriorated earlier than the secondelectrode 16. When the current value when the first electrode 15 servesas an anode is set so as to be smaller than the current value when thesecond electrode 16 serves as an anode while the duty ratio is notchanged in accordance with the cycle pattern, a change in luminance atthe lighting frequency occurs and is recognized as flicker of the lightsource itself, or as the lighting frequency approaches the drivingfrequency of the display device, the change in luminance is recognizedas luminance irregularity (scroll noise) due to interference.

FIG. 10 is a conceptual view illustrating the structure of a projectorincorporated with the light source device 100 of FIG. 1. A projector 200includes a light source device 100, an illumination optical system 20, acolor separation optical system 30, a light modulation section 40, across dichroic prism 50, and a projection lens 60. The light modulationsection 40 includes three liquid crystal light valves 40 a, 40 b, and 40c having the same structure.

In the projector 200, the light source device 100 includes the lightsource unit 10 and the light source driving device 70 shown in FIG. 1.The light source device 100 generates illumination light forilluminating the light modulation section 40, that is, the liquidcrystal light valves 40 a, 40 b, and 40 c, through the illuminationoptical system 20.

The illumination optical system 20 includes a parallelizing lens 22 thatparallelizes the direction of a light beam emitted from the lightsource, first and second fly-eye lenses 23 a and 23 b that constitute anintegrator optical system for dividing light into partial light beamsand superimposing the partial light beams, a polarization conversionelement 24 that arranges the light polarization direction, asuperimposing lens 25 that superimposes light having passed through bothfly-eye lenses 23 a and 23 b, and a mirror 26 that bends the opticalpath of light. In the illumination optical system 20, the parallelizinglens 22 converts the light beam direction of illumination light emittedfrom the light source unit 10 into substantially parallel light. Thefirst and second fly-eye lenses 23 a and 23 b each include a pluralityof element lenses arranged in a matrix. The element lenses constitutingthe first fly-eye lens 23 a divide light having passed through theparallelizing lens 22 into partial light beams, and collect the partiallight beams separately. The element lenses constituting the secondfly-eye lens 23 b convert the partial light beams from the first fly-eyelens 23 a into light beams having an appropriate divergence angle. Thepolarization conversion element 24 has an array of PBS, mirror,retardation film, and the like as a set of elements, and has a functionof converting the partial light beams divided by the first fly-eye lens23 a into one-directional linear polarized light. The superimposing lens25 appropriately converges illumination light having passed through thepolarization conversion element 24 as a whole such that illuminationlight can be superimposed on regions to be illuminated of the liquidcrystal light valves 40 a, 40 b, and 40 c of a subsequent stage as lightmodulation devices for respective colors. That is, illumination lighthaving passed through both fly-eye lenses 23 a and 23 b and thesuperimposing lens 25 passes through the color separation optical system30, which will be described below in detail, and is superimposed anduniformly illuminates liquid crystal panels 41 a, 41 b, and 41 c forrespective colors provided in the light modulation section 40.

The color separation optical system 30 includes first and seconddichroic mirrors 31 a and 31 b, reflecting mirrors 32 a, 32 b, 32 c, andthree field lenses 33 a, 33 b, and 33 c. The color separation opticalsystem 30 separates illumination light from the illumination opticalsystem 20 into three color light components of red (R), green (G), andblue (B), and introduces the color light components to the liquidcrystal light valves 40 a, 40 b, and 40 c of a subsequent stage,respectively. More specifically, first, the first dichroic mirror 31 atransmits the R light component of the three R, G, and B lightcomponents, and reflects the G and B light components. The seconddichroic mirror 31 b reflects the G light component of the two G and Blight components, and transmits the B light component. Next, in thecolor separation optical system 30, the R light component having passedthrough the first dichroic mirror 31 a passes through the reflectingmirror 32 a and enters the field lens 33 a for controlling the incidentangle. The G light component reflected by the first dichroic mirror 31 aand further reflected by the second dichroic mirror 31 b enters thefield lens 33 b for controlling the incident angle. The B lightcomponent having passed through the second dichroic mirror 31 b passesthrough relay lenses LL1 and LL2 and the reflecting mirrors 32 b and 32c, and enters the field lens 33 c for controlling the incident angle.

The liquid crystal light valves 40 a, 40 b, and 40 c constituting thelight modulation section 40 are non-emission-type light modulationdevices for modulating the spatial intensity distribution of incidentillumination light. The liquid crystal light valves 40 a, 40 b, and 40 cinclude three liquid crystal panels 41 a, 41 b, and 41 c thatcorrespondingly receive the color light components emitted from thecolor separation optical system 30, three first polarizing filters 42 a,42 b, and 42 c that are disposed on the incident sides of the liquidcrystal panels 41 a, 41 b, and 41 c, respectively, and three secondpolarizing filters 43 a, 43 b, and 43 c that are disposed on theemission sides of the liquid crystal panels 41 a, 41 b, and 41 c,respectively. The R light component having passed through the firstdichroic mirror 31 a enters the liquid crystal light valve 40 a throughthe field lens 33 a to illuminate the liquid crystal panel 41 a of theliquid crystal light valve 40 a. The G light component reflected by thefirst and second dichroic mirrors 31 a and 31 b enters the liquidcrystal light valve 40 b through the field lens 33 b to illuminate theliquid crystal panel 41 b of the liquid crystal light valve 40 b. The Blight component having been reflected by the first dichroic mirror 31 aand passed through the second dichroic mirror 31 b enters the liquidcrystal light valve 40 c through the field lens 33 c to illuminate theliquid crystal panel 41 c of the liquid crystal light valve 40 c. Theliquid crystal panels 41 a to 41 c modulate the spatial intensitydistribution of incident illumination light in the polarizationdirection to control the polarization states of the three color lightcomponents having entered the liquid crystal panels 41 a to 41 c foreach pixel in accordance with driving signals or image signals input aselectrical signals to the liquid crystal panels 41 a to 41 c. In thiscase, the first polarizing filters 42 a to 42 c control the polarizationdirection of illumination light entering the liquid crystal panels 41 ato 41 c, respectively. The second polarizing filters 43 a to 43 cextract modulated light having a predetermined polarization directionfrom modulated light emitted from the liquid crystal panels 41 a to 41c. In this way, the liquid crystal light valves 40 a, 40 b, and 40 cform image light for the respective colors.

The cross dichroic prism 50 synthesizes image light for the respectivecolors from the liquid crystal light valves 40 a, 40 b, and 40 c. Morespecifically, the cross dichroic prism 50 has a substantially squareshape in plan view formed by affixing four rectangular prisms, and apair of dielectric multilayer films 51 a and 51 b crossing in an X shapeare formed on the boundaries of the affixed rectangular prisms. Thefirst dielectric multilayer film 51 a reflects the R light component,and the second dielectric multilayer film 51 b reflects the B lightcomponent. The cross dichroic prism 50 reflects the R light componentfrom the liquid crystal light valve 40 a by the dielectric multilayerfilm 51 a so as to be emitted to the right with respect to the traveldirection. The cross dichroic prism 50 directs the G light componentfrom the liquid crystal light valve 40 b so as to advance straight andto be emitted through the dielectric multilayer films 51 a and 51 b. Thecross dichroic prism 50 reflects the B light component from the liquidcrystal light valve 40 c by the dielectric multilayer film 51 b so as tobe emitted to the left with respect to the travel direction. In thisway, the cross dichroic prism 50 synthesizes the R, G, and B lightcomponents to produce synthesized light as image light for forming acolor image.

The projection lens 60 is a projection optical system which enlargesimage light on a desired scale of enlargement as synthesized light fromthe cross dichroic prism 50, and projects a color image onto a screen(not shown).

According to the projector 200, a pair of electrodes 15 and 16constituting the light source device 100 can be alternately repaired,and one of the pair of electrodes 15 and 16 can be prevented from beingdeteriorated unevenly and early. Therefore, the projection luminance ofthe projector 200 can be maintained over a long period of time.

The invention is not limited to the foregoing embodiment, but variousmodifications may be made without departing from the scope of theinvention. For example, the following modifications may be made.

For example, the modulation patterns shown in FIGS. 5A to 5C, 6A to 6C,and 7A to 7C are just examples, and an AC current to be supplied to apair of electrodes 15 and 16 may be changed in accordance with variousmodulation patterns. In this case, the convection AF can be preventedfrom being excessively localized inside the light-emitting tube 1 whilethe first and second electrodes 15 and 16 can be repaired. In addition,one of the pair of electrodes 15 and 16 can be prevented from beingdeteriorated unevenly and early.

In the foregoing embodiment, a case in which the auxiliary mirror 3 isprovided, and accordingly the first electrode 15 becomes higher than thesecond electrode 16 in temperature has been described. Even if noauxiliary mirror 3 is provided, in an air-cooled state, a difference intemperature may occur between both electrodes 15 and 16, or if bothelectrodes 15 and 16 are different in size, a difference in temperaturemay occur between both electrodes 15 and 16. In this case, by using thewaveforms shown in FIGS. 5A to 5C, 6A to 6C, and 7A to 7C, currentdriving can be achieved with the difference in temperature between bothelectrodes 15 and 16 compensated.

As the lamp for the light source unit 10 of the foregoing embodiment,various kinds of lamps, such as a high-pressure mercury lamp, a metalhalide lamp, or the like, may be used.

In the projector 200 of the foregoing embodiment, in order to separatelight from the light source device 100 into a plurality of partial lightbeams, a pair of fly-eye lenses 23 a and 23 b are used, but theinvention may be applied to a projector in which no fly-eye lens, thatis, no lens array is used. In addition, the fly-eye lenses 23 a and 23 bmay be substituted with a rod integrator.

The projector 200 uses the polarization conversion element 24 thatconverts light from the light source device 100 into polarized light ina specific direction, but the invention may be applied to a projector inwhich no polarization conversion element 24 is used.

In the foregoing embodiment, an example where the invention is appliedto a transmission type projector has been described, but the inventionmay be applied to a reflection type projector. The term “transmissiontype” herein means a liquid crystal light valve including a liquidcrystal panel and the like transmits light, and the term “reflectiontype” means that a liquid crystal light valve reflects light. The lightmodulation device is not limited to a liquid crystal panel. For example,a light modulation device using a micro mirror may be used.

There are a front type projector that projects an image from theprojection surface viewing side, and a rear type projector that projectsan image from the side opposite to the projection surface viewing side.The configuration of the projector shown in FIG. 10 may be applied toboth types.

In the foregoing embodiment, only an example of the projector 200 whichuses the three liquid crystal panels 41 a to 41 c has been described,but the invention may be applied to a projector which uses a singleliquid crystal panel, a projector which uses two liquid crystal panels,or a projector which uses four or more liquid crystal panels.

In the foregoing embodiment, the color light components are modulated byusing the color separation optical system 30 and the liquid crystallight valves 40 a, 40 b, and 40 c. Alternatively, the color lightcomponents may be modulated and synthesized by using a combination of acolor wheel which is illuminated by the light source device 100 and theillumination optical system 20, and a device which includes pixels ofmicro mirrors and onto which light having passed through the color wheelis irradiated.

The entire disclosure of Japanese Patent Application No. 2008-49293,filed Feb. 29, 2008 is expressly incorporated by reference herein.

1. A method for driving a discharge lamp that supplies an AC current toa discharge lamp having a first electrode and a second electrode so asto produce discharge and to cause the discharge lamp to emit light,comprising the steps of: in a situation where a tip portion of the firstelectrode becomes higher than a tip portion of the second electrode intemperature when power of the same amount is fed to the first and secondelectrodes during a steady operation in which the AC current is suppliedto the first electrode and the second electrode, changing the duty ratioof the AC current to be supplied between the first electrode and thesecond electrode in accordance with a predetermined pattern, and settinga current value when the first electrode operates as an anode during onecycle so as to be smaller than a current value when the second electrodeoperates as an anode during one cycle.
 2. The method according to claim1, wherein a primary reflecting mirror is disposed on the secondelectrode side to reflect a light beam generated by discharge betweenthe first electrode and the second electrode so as to be emitted towarda region to be illuminated, and an auxiliary reflecting mirror isdisposed on the first electrode side so as to be opposite the primaryreflecting mirror to reflect a light beam from an inter-electrode spacebetween the first electrode and the second electrode toward theinter-electrode space.
 3. The method according to claim 1, wherein theAC current constantly provides a difference of a predetermined currentvalue between the absolute value of the current value when the firstelectrode operates as an anode during one cycle and the absolute valueof the current value when the second electrode operates as an anodeduring one cycle.
 4. The method according to claim 1, wherein the ACcurrent constantly provides a difference of a predetermined ratiobetween the absolute value of the current value when the first electrodeoperates as an anode during one cycle and the absolute value of thecurrent value when the second electrode operates as an anode during onecycle.
 5. The method according to claim 1, wherein the current value iscontrolled such that the average power value during one cycle of the ACcurrent substantially becomes identical to the average power valueduring one cycle of the predetermined pattern.
 6. The method accordingto claim 5, wherein the current value is controlled only at a polarityhaving a larger duty ratio during one cycle of the AC current.
 7. Adriving device that supplies an AC current to a discharge lamp having afirst electrode and a second electrode so as to produce discharge and tocause the discharge lamp to emit light, the driving device comprising: acurrent driving circuit that, in a situation where a tip portion of thefirst electrode becomes higher than a tip portion of the secondelectrode in temperature when power of the same amount is fed to thefirst and second electrodes during a steady operation in which the ACcurrent is supplied to the first electrode and the second electrode,changes the duty ratio of the AC current to be supplied between thefirst electrode and the second electrode in accordance with apredetermined pattern, and sets the absolute value of a current valuewhen the first electrode operates as an anode during one cycle so as tobe smaller than the absolute value of a current value when the secondelectrode operates as an anode during one cycle.
 8. A projectorcomprising: a light source device that is driven by the driving methodaccording to claim 1 and emits light; a light modulation device thatreceives a light beam from the light source device; and a projectionoptical system that projects an image formed by the light modulationdevice.
 9. A projector comprising: a light source device that is drivenby the driving method according to claim 2 and emits light; a lightmodulation device that receives a light beam from the light sourcedevice; and a projection optical system that projects an image formed bythe light modulation device.
 10. A projector comprising: a light sourcedevice that is driven by the driving method according to claim 3 andemits light; a light modulation device that receives a light beam fromthe light source device; and a projection optical system that projectsan image formed by the light modulation device.
 11. A projectorcomprising: a light source device that is driven by the driving methodaccording to claim 4 and emits light; a light modulation device thatreceives a light beam from the light source device; and a projectionoptical system that projects an image formed by the light modulationdevice.
 12. A projector comprising: a light source device that is drivenby the driving method according to claim 5 and emits light; a lightmodulation device that receives a light beam from the light sourcedevice; and a projection optical system that projects an image formed bythe light modulation device.
 13. A projector comprising: a light sourcedevice that is driven by the driving method according to claim 6 andemits light; a light modulation device that receives a light beam fromthe light source device; and a projection optical system that projectsan image formed by the light modulation device.
 14. A projectorcomprising: a light source device that includes a discharge lamp havinga first electrode and a second electrode and emits light; a drivingdevice according to claim 7; a light modulation device that receives alight beam from the light source device; and a projection optical systemthat projects an image formed by the light modulation device.